1. Field of the Invention
The invention relates to methods and apparatus for sensing and analyzing electrical activity in a patient's brain. In particular, the invention relates to sensing an electrical action potential generated in a selected portion of a patient's brain and comparing a waveform of the electrical action potential to one or more predetermined waveforms.
2. Background of the Related Art
It is known that different regions of the brain are used to control different parts of the body and to process different sensory inputs. It is also known that when a human performs a certain function, such as moving an extremity or listening to a particular sound, a discrete region of the brain will generate electrical action potentials to accomplish that function. It is also known that direct electrical stimulation of a particular region of the brain can cause at least partial reproduction of the functions or sensory input normally associated with that region of the brain.
Determining which portions of a patient's brain are responsible for certain motor activities or certain sensory functions has become known as brain "mapping." After a patient's brain has been mapped, the brain can be electrically stimulated to restore lost functions.
For instance, it is possible to determine which portions of a patient's brain are responsible for processing particular sound frequencies. Once a neurosurgeon knows which portions of the patient's brain are responsible for processing each sound frequency, it is possible to electrically stimulate selected portions of the patient's brain to cause the patient to "hear" particular sounds. Thus, a patient whose hearing has been partially or permanently damaged can be made to hear again if an electrical prosthetic device is used to sense sounds and to electrically stimulate the brain to perceive those sounds.
Brain mapping is typically carried out with a penetrating microelectrode, such as the one shown in FIG. 1. The microelectrode is inserted into a patient's brain to sense electrical action potentials. The microelectrode includes a longitudinal support 226 having a first end 206a and a second end 206b. The microelectrode also includes a plurality of electrical contacts 220 formed along the longitudinal support 226 of the microelectrode. One or more wires 232, connect the plurality of electrical contacts 220 to corresponding leads 238. The electrical leads 238 are then connected to a device which is capable of sensing a voltage generated adjacent the electrical contacts 220, or of applying a voltage to the contacts 220.
To map a portion of a patient's brain, a microelectrode like the one shown in FIG. 1 is inserted through a burr hole into a portion 150 of a patient's brain, as shown in FIG. 2. The electrical contacts 220 on the microelectrode 200 are connected to one or more sensors 426 via an electrical cable 424. If, for example, functions relating to movement of the patient's arm are being mapped, the patient is then instructed to move his arm in some manner and the electrical action potentials that are produced by the patient's brain are sensed by the sensors 426, via the electrical contacts 220 of the microelectrode 200. Typically, only certain ones of the electrical contacts 220 will sense an electrical action potential. This indicates that the portions of the patient's brain adjacent the electrical contacts 220 sensing electrical action potentials are responsible for the particular movement of the patient's arm. The patient can then be instructed to move his arm in a different manner, and the electrical action potentials generated by this movement are also sensed by the microelectrode 200. In this manner, a neurosurgeon can determine exactly which portions of the patient's brain are responsible for particular types of arm movements.
At this point, the neurosurgeon could selectively excite certain electrical contacts 220 of the microelectrode 200 via exciters 428 connected to the microelectrode via a cable 414. It is sometimes possible for electrical excitation of selected portions of the patient's brain to reproduce particular types of arm movements.
A similar process can be followed to determine which portions of a patient's brain are responsible for processing certain frequency sounds. The patient's brain can then be electrically excited to cause the patient to "hear" certain sounds. Likewise, this process can be used to determine which portions of a patient's brain are responsible for processing sensory inputs from other sensory organs such as the eyes, the nose, or from touch receptors.
FIG. 3 shows a typical system used to help map a patient's brain. The device includes a microelectrode 200, such as the one shown in FIG. 1, an amplifier 230, a signal processor 240, and a transducer 250. Individual electrical contacts 220 of the microelectrode 200 are connected to the amplifier 230, which amplifies any electrical action potentials generated in the patient's brain adjacent the electrical contacts 220. The amplified signals are then fed to a signal processor 240, which processes the amplified signals to determine whether the amplitude of the signals rises above a certain threshold level. When the amplitude of a sensed electrical action potential exceeds a predetermined threshold, the signal processor causes a transducer 250 to generate a user recognizable signal, such as an audible tone or the illumination of a lamp. Such a system is commonly known as an amplitude threshold discriminator.
FIG. 4 shows a plot of sensed electrical action potential voltage at a particular electrical contact 220 of a microelectrode 200 over time. The dashed line TH indicates a threshold electrical voltage. If a system like the one shown in FIG. 3 is used to sense the electrical action potential voltages shown in FIG. 4, whenever the voltage of the electrical action potential exceeds the threshold voltage TH, the transducer 250 of the system would output a user recognizable signal. For instance, at times T1, T5 and T8, the sensed electrical action potential voltage does not exceed the threshold voltage, and no user recognizable signal would be output. However, at times T2-T4, T6, T7 and T9, the sensed electrical action potential voltage exceeds the threshold voltage, and the transducer 250 of the system would output a user recognizable signal.
The prior art system described above cannot always accurately map a patient's brain because simply determining whether the voltage of an electrical action potential exceeds a threshold voltage does not provide enough information about the electrical activity occurring in the brain to accurately map the brain. Although an amplitude threshold discriminator, as described above, can eliminate some background noise, the system provides poor resolution and is incapable of accurately determining that a portion of a patient's brain is responsible for a certain activity if the electrical action potential associated with that activity does not exceed the threshold voltage.